1. Field of the Invention
The present invention relates to a magnetic recording medium and particularly relates to improvements of a magnetic recording medium being excellent in durability and magnetic characteristics.
2. Description of the Related Art
Presently, a magnetic recording medium such as a magnetic tape used for video tape recorders (referred to as VTR) is demanded to have high recording density and high reliability when the magnetic tape is recorded or reproduced. For the purpose of improving durability of the magnetic recording medium, various non-magnetic particles are added in a magnetic coat of the magnetic recording medium and finer particles of ferromagnetic powder are employed to increase packing density thereof.
Major non-magnetic particles above include an antistatic agent used for reducing cohesion of a magnetic tape to guide posts of a VTR while the tape is running, and abrasives used for preventing heads of the VTR from defective recording or reproducing due to dirt of particles dropped off a magnetic coat. However, application of these non-magnetic particles into a magnetic coat causes a decrease of the packing density of ferromagnetic powder, in turn, impedes a demand for high output of the magnetic tape. Such as alumina or the like powders are commonly used as abrasives, but they have following problems although the alumina with its hardness and shape is effective to maintain abrasiveness of a magnetic recording medium. Alumina powder as well as ferromagnetic powder are improved for finer grains. However, the finer grains make it difficult to disperse the powder uniformly into a magnetic coat and degrade magnetic characteristics and reliability.
Accordingly, to improve the above mentioned problems, there provided an improved polyurethane resin which is commonly used for a magnetic recording medium as a binder, wherein the polyurethane resin is improved in functional polar group. For example, a binder composed of a polyurethane resin, which is reacted with metal base sulfonic acid and tertiary amine, is proposed in the Japanese Patent Publication No. S58-41565 and the Japanese Patent Application Laid-open Publication No. H1-91315. However, such binders are still desired to be improved since alumina powder is poor in surface adsorption with resin.
Furthermore, the proposed medium is unsatisfactory as a desirable magnetic recording medium since alumina powder is easily dropped out.
It is difficult to obtain both durability and magnetic characteristics of a magnetic recording medium by a combination of alumina powder as abrasives and polyurethane resin as a binder.